Advances in conventional cancer therapy have not produced any significant improvement in the treatment of glioblastoma multiforme (GBM). Therefore, the further development of novel treatment modalities for GBM, such as boron neutron capture therapy (BNCT), remains a pressing task for the health science community in the USA. Subject of the present grant application is the continued development of 3-carboranyl thymidine analogues (3CTAs), a novel class of BNCT agents for the treatment of GBM. The mechanism of action of 3CTAs is primarily based on their ability to function as substrates of human thymidine kinase 1 (TK1), which is only active in proliferating cells. BNCT of tumor-bearing rodents that had received N5-2OH, the current lead compound of our 3CTA library, via intracerebral (i.c.) or intratumoral (i.t.) injection resulted in reduced tumor growth and prolonged survival times. On the other hand, the same studies have revealed two major limitations of N5-2OH and other 3CTAs. These are (1) the suboptimal competition with endogenous thymidine (Thd) at the substrate-binding site of TK1 and (2) the lack of water-solubility, which necessitated the use of organic solvents to solubilize 3CTAs for biological studies. In addition crucial biochemical and biological features of 3CTAs are still unknown. These are (3) the anabolism of 3CTAs beyond the monophosphorylation step, including their possible incorporation into DNA, and (4) the mechanisms that 3CTAs and their metabolites recruit to enter and exit cells. To address these shortcomings, the following SPECIFIC AIMS were developed: (1) To synthesize 3CTAs with improved ability to compete with Thd at the active site of TK1, to evaluate their substrate and inhibitory capacities in enzyme assays with TK1, and to determine their physicochemical properties, (2) to synthesize water-soluble amino acid prodrugs of N5-2OH and to evaluate their applicability for i.c. administration, (3) To study the metabolism of N5-2OH and 3CTAs with improved drug-like and enzymatic properties that emerged from Specific Aims 1 and 2 in vitro and in enzyme assays with nucleotide kinases and DNA polymerase, (4) to study the mechanisms of cellular influx and efflux of N5-2OH, 3CTAs with improved drug-like and enzymatic properties that emerged from Specific Aims 1 and 2, and their metabolites in vitro, and (5) to optimize parameters for i.c. administration of the 3CTA with the most improved drug-like and enzymatic properties that emerged from Specific Aims 1- 4 to RG2 glioma bearing rats and to determine possible neurotoxicity.